The present invention relates to a particle size measuring cell and, more particularly, to a measuring cell in which the sizes of particles entrained in a liquid stream are measured by passing the liquid stream through a laser beam and detecting the light scattered from the laser beam.
In known particle size measuring cells of the above type, individual particles in the liquid stream passing through the laser beam scatter the light toward a photodetector, which transmits a signal in response. The amount of scattered light impinging on the photodetector is proportional to the size of the particle causing the scattering, and the amplitude of the signal transmitted by the photodetector is determined by the amount of impinging light, so that the amplitude of the signal transmitted by the photodetector is a measure of the size of the particle passing through the laser beam.
The scattered light is collected by a lens and focused on the photodetector, which generates its pulses in response to each particle passing through the laser beam. The liquid stream is typically carried through the laser beam by a conduit of transparent material. As a result, there are substantial losses in the energy of the beam as it passes from air through one wall of the conduit, the fluid stream, and then the other wall of the conduit, thus, passing through numerous interfaces between various media having different indexes of refraction. Furthermore, since the conduits are typically circular in cross-section, the fluid stream is circular in cross-section and, therefore, fairly thick at the center where the laser beam intersects, thereby increasing the losses. For these reasons, prior devices for detecting and measuring particles in liquid streams have been limited to use with a laser beam emitted from a conventional laser. An external cavity laser has not been used in connection with measuring particles in liquid streams since losses in the light beam as it passed through the liquid stream have rendered it difficult, if not impossible, to maintain lasing.